Abstract: We present here the discovery and characterisation of a very light planet
around HD4308. The planet orbits its star in 15.56 days. The circular
radial-velocity variation presents a tiny semi-amplitude of 4.1 m/s that
corresponds to a planetary minimum mass m2sin(i)=14.1 Earth masses. The planet
was unveiled by high-precision radial-velocity measurements obtained with the
HARPS spectrograph on the ESO 3.6-m telescope. The radial-velocity residuals
around the Keplerian solution are 1.3 m/s, demonstrating the very high quality
of the HARPS measurements. Activity and bisector indicators exclude any
significant perturbations of stellar intrinsic origin, which supports the
planetary interpretation. Contrary to most planet-host stars, HD4308 has a
marked sub-solar metallicity ([Fe/H]=-0.31), raising the possibility that very
light planet occurrence might show a different coupling with the parent star's
metallicity than do giant gaseous extra-solar planets. Together with
Neptune-mass planets close to their parent stars, the new planet occupies a
position in the mass-separation parameter space that is constraining for
planet-formation and evolution theories. The question of whether they can be
considered as residuals of evaporated gaseous giant planets, ice giants, or
super-earth planets is discussed in the context of the latest core-accretion
models.